1. Field of the Invention
The invention relates to a hydraulic control system for an automatic transmission, and, more specifically, to an automatic transmission control system of the type utilizing a hydraulic control circuit to which an accumulator is incorporated.
2. Description of Related Art
Automatic transmissions used in automotive applications include a transmission gear mechanism which is driven by input torque from an engine through a torque converter. The torque transmission path within the transmission gear mechanism is changed by selectively locking and unlocking a plurality of friction coupling elements, such as clutches and brakes, to automatically place the automatic transmission in desired gears according to driving conditions. This type of automatic transmission is equipped with a hydraulic control circuit which manages the supply of hydraulic operating pressure to the friction coupling elements.
Hydraulic control circuit includes a hydraulic pressure source, such as a regulator valve which develops line pressure as source pressure for the hydraulic operating pressure and a duty solenoid valve which directly generates hydraulic operating pressure to be delivered to the friction coupling elements, shift valves which serve as a switching means to selectively direct the hydraulic operating pressure between the friction coupling elements. In order for the hydraulic control circuit to promote a gentle rise in pressure level of the hydraulic operating pressure for the friction coupling elements, or in order for the hydraulic control circuit to absorb fluctuations in the hydraulic operating pressure in the case where the hydraulic operating pressure is generated directly by the duty solenoid valve, an accumulator is further incorporated in the hydraulic control circuit between the hydraulic pressure source and the friction coupling elements. This spring loaded accumulator shifts a piston against a spring in response to the hydraulic operating pressure.
Because effective operating pressures needed by the respective friction coupling elements are not at the same highest level, operating characteristics of the accumulators must be different to comply with the specific hydraulic operating demands of the respective friction coupling elements. As a specific example, since a band brake used in a second gear is applied in a wide range of engine throttle operating conditions from a full throttle condition to a throttled-down condition, the accumulator for the band brake must be able to operate reliably with high hydraulic operating pressure. Conversely, since a low-reverse brake, which is used in a reverse gear or in a first gear serving as engine brake in a low (L) range, is applied when the engine throttle opening is relatively small, the accumulator utilized for the low-reverse brake must be able to operate reliably with relatively low hydraulic operating pressure.
An automatic transmission control system, such as known in U.S. Pat. No. 4,875,391, which accommodates such demands for operating characteristics, is equipped with separate accumulators for the respective friction coupling elements in a hydraulic control circuit. In this prior art automatic transmission control system, as a result of the utilization of separate accumulators for the respective friction coupling elements, an increased number of accumulators must be installed in a control valve unit, which always poses certain design problems in regard to space and layout, and makes it difficult to provide a compact design of control valve unit for integration into the automatic transmission.
It has been considered to utilize a single accumulator of a type capable of providing a sufficient piston stroke at low hydraulic operating pressure while providing reliable operation with high hydraulic operating pressure, and having the ability to effectively absorb fluctuations in hydraulic operating pressure, which can accommodate the demands for hydraulic operating pressure at different highest levels commonly to more-than-one frictional coupling elements. However, a compression spring utilized in this type of accumulator must have a long physical dimension with a relatively small spring constant which imposes various dimensional constraints on the accumulator. If this type of accumulator is to be integrated into the control valve unit, it would still not solve the design problems in regard to the size and layout of the control valve unit.